Wavelength conversion laser device

ABSTRACT

A wavelength conversion laser device includes a solid-state laser element having a waveguide structure including a laser medium that amplifies laser beams by providing a gain generated due to absorption of pump light to the laser beams and outputs a fundamental wave, and a wavelength conversion element having a waveguide structure including a nonlinear optical material that converts a part of a fundamental wave output from the solid-state laser element to a second harmonic, to resonate the fundamental wave by an optical resonator structure including the solid-state laser element and the wavelength conversion element and outputs a second harmonic from the wavelength conversion element. The solid-state laser element outputs a linearly polarized fundamental wave, and differentiates a polarization state of a fundamental wave having passed through the wavelength conversion element and entering into the solid-state laser element from linear polarization output from the solid-state laser element, so that wavelength conversion efficiency of the wavelength conversion element is not decreased in a peak wavelength of a gain band.

TECHNICAL FIELD

The present invention relates to a wavelength conversion laser devicethat outputs laser beams of a predetermined wavelength by performingwavelength conversion of a fundamental wave generated by a laser medium.

BACKGROUND ART

Recently, as a light source of an optical information processingtechnology, for example, research and development of a visible lightlaser such as green light laser and blue light laser are under way. As atype of visible light lasers, a wavelength conversion laser device thatapplies a wavelength conversion technique to make near infrared laserbeams have a shorter wavelength has been known. Generally, in thewavelength conversion laser device, a wavelength conversion element madeof a nonlinear optical material is provided inside or outside of anoptical resonator of a semiconductor laser or a solid-state laser, andlaser beams (fundamental waves) generated by the optical resonator arepropagated to the wavelength conversion element, thereby outputting asecond harmonic, which is wavelength-converted to half a wavelength (adouble frequency) with respect to the fundamental waves.

At this time, an oscillation wavelength bandwidth of the opticalresonator needs to be matched with a phase matching width of thewavelength conversion element. However, the phase matching width of thewavelength conversion element is generally very narrow, and an output ofthe wavelength conversion laser device fluctuates due to its externalenvironment. Therefore, to achieve wavelength conversion by thewavelength conversion element in a highly efficient manner, a coherentlight source has been proposed as a wavelength conversion laser devicehaving a configuration such that a light source wavelength is fixedwithin a tolerance of the wavelength conversion element so that it isnot much affected by a change of the external environment (see, forexample, Patent Document 1). In this conventional coherent light source,fundamental waves from the laser medium are converted to a harmonic bythe wavelength conversion element, and the fundamental waves reflectedby a reflector are returned to the laser medium, thereby fixing theoscillation wavelength of the laser medium to the wavelength of returnlight so that the oscillation wavelength of the laser medium isautomatically fixed to a phase matching wavelength of the wavelengthconversion element.

-   Patent Document 1: Japanese Patent Application Laid-open No.    2006-19603

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In a wavelength conversion laser device of an internal wavelengthconversion type including a laser medium and a wavelength conversionelement in a pair of resonator mirrors, when internal wavelengthconversion is performed by using a laser medium having a wider gain bandthan a phase matching band (a wavelength conversion band) of thewavelength conversion element, the fundamental waves are firstlaser-oscillated at a peak wavelength of the gain band, and wavelengthconversion is performed by the wavelength conversion element at the peakwavelength of the gain band. However, a loss in the optical resonator atthe peak wavelength of the gain band increases due to the wavelengthconversion, thereby causing laser-oscillation at the wavelength of thegain band outside the phase matching band. As a result, the fundamentalwaves in the phase matching band decrease, thereby causing a problemthat wavelength conversion efficiency by the wavelength conversionelement is deteriorated. Furthermore, in Patent Literature 1, theoscillation wavelength of the laser medium is automatically fixed to thephase matching wavelength of the wavelength conversion element. However,as described above, because the phase matching band of the wavelengthconversion element is generally narrower than the oscillation wavelengthband (the gain band) of the laser medium, it has not been possible tosuppress the laser-oscillation outside the phase matching band.

In view of the above problems, an object of the present invention is toachieve a wavelength conversion laser device in which even whenlaser-oscillation occurs at a peak wavelength of a gain band of a lasermedium to perform wavelength conversion, wavelength conversionefficiency by a wavelength conversion element at the peak wavelength ofthe gain band does not decrease. Another object of the present inventionis to achieve a wavelength conversion laser device in which, withoutadding any optical parts or large-scale optical parts, wavelengthconversion efficiency by a wavelength conversion element at a peakwavelength of a gain band does not decrease.

Means for Solving Problem

In order to attain the above object, in a wavelength conversion laserdevice of the present invention including a solid-state laser elementhaving a waveguide structure including a laser medium that amplifieslaser beams by providing a gain generated due to absorption of pumplight to the laser beams and outputs a fundamental wave and a wavelengthconversion element having a waveguide structure including a nonlinearoptical material that converts a part of a fundamental wave output fromthe solid-state laser element to a second harmonic, the wavelengthconversion laser device resonates a fundamental wave by an opticalresonator structure including the solid-state laser element and thewavelength conversion element and outputs a second harmonic from thewavelength conversion element, and the solid-state laser element outputsa linearly polarized fundamental wave. Additionally, the wavelengthconversion laser device further includes a filter unit thatdifferentiates a polarization state of a fundamental wave having passedthrough the wavelength conversion element and entering into thesolid-state laser element from linear polarization output from thesolid-state laser element.

EFFECT OF THE INVENTION

According to the present invention, an oscillation wavelength width offundamental wave is made narrow near a peak wavelength of a gain band ofa laser medium, to be substantially the oscillation wavelength width ofthe fundamental wave. Accordingly, even when wavelength conversion by awavelength conversion element is performed at the peak wavelength of thelaser medium, and a loss in an optical resonator at the peak wavelengthincreases, laser-oscillation of the fundamental wave outside thewavelength conversion band (substantially equal to the oscillationwavelength width of the fundamental wave) does not occur. As a result,efficient wavelength conversion of the fundamental wave can be performedin the wavelength conversion band of the wavelength conversion element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically depicting a configuration ofa wavelength conversion laser device according to a first embodiment ofthe present invention.

FIG. 2 depicts a relation between a field strength and an axialdirection when laser beams having linear polarization in a c-axisdirection travel back and forth in a nonlinear optical material.

FIG. 3A depicts installation angle θ dependency of MgO:PPLN of a degreeof polarization of laser beams in the c-axis direction after passingthrough MgO:PPLN.

FIG. 3B depicts installation angle θ dependency of MgO:PPLN of a degreeof polarization of laser beams in the c-axis direction after passingthrough MgO:PPLN.

FIG. 4A depicts a wavelength dependency of a strength of polarization ina-axis and c-axis directions and a degree of polarization in the c-axisdirection of laser beams after passing through MgO:PPLN.

FIG. 4B depicts a wavelength dependency of a strength of polarization inthe a-axis and c-axis directions and a degree of polarization in thec-axis direction of laser beams after passing through MgO:PPLN.

FIG. 4C depicts a wavelength dependency of a strength of polarization inthe a-axis and c-axis directions and a degree of polarization in thec-axis direction of laser beams after passing through MgO:PPLN.

FIG. 4D depicts a wavelength dependency of a strength of polarization inthe a-axis and c-axis directions and a degree of polarization in thec-axis direction of laser beams after passing through MgO:PPLN.

FIG. 5A is a schematic diagram of a relation between a gain shape withrespect to a wavelength of a fundamental wave and a gain band of a lasermedium.

FIG. 5B is a schematic diagram of a relation between a gain shape withrespect to a wavelength of a fundamental wave and a gain band of a lasermedium.

FIG. 6 is a schematic diagram of a shift of a wavelength conversion bandand a fundamental wave in a nonlinear optical material caused by atemperature change.

FIG. 7 is a perspective view schematically depicting a configuration ofa wavelength conversion laser device according to a second embodiment ofthe present invention.

FIG. 8 is a perspective view schematically depicting a configuration ofa wavelength conversion laser device according to a third embodiment ofthe present invention.

FIG. 9 depicts wavelength dependency of an intensity of polarization inan a-axis direction and a degree of polarization in a c-axis directionof laser beams after passing through MgO:PPLN and a ¼ wavelength plate.

EXPLANATIONS OF LETTERS OR NUMERALS

-   -   10, 10A, 10B wavelength conversion laser device    -   11 semiconductor laser    -   12 solid-state laser element    -   13 wavelength conversion-filter element    -   13A wavelength conversion element    -   14 ½ wavelength plate    -   15 ¼ wavelength plate    -   111, 123 a, 123 b, 133 a, 133 b, 151 facet    -   121 laser medium    -   122, 132 clad    -   131 nonlinear optical material

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Exemplary embodiments of a wavelength conversion laser device accordingto the present invention will be explained below in detail withreference to the accompanying drawings. The present invention is notlimited to the embodiments. In addition, perspective views of awavelength conversion laser device in the following embodiments areshown schematically, and a relation between the thickness and width of alayer and a ratio of the thickness of respective layers are differentfrom actual products.

First Embodiment

FIG. 1 is a perspective view schematically depicting a configuration ofa wavelength conversion laser device according to a first embodiment ofthe present invention. It is assumed that a direction of R shown in FIG.1 is an optical axis indicating an oscillating direction of laser beams.A wavelength conversion laser device 10 includes a semiconductor laser11 as an pump light source, a solid-state laser element 12 that performsoscillation as well as amplification of laser beams, which becomefundamental waves, using laser beams output from the semiconductor laser11 as pump light, and a wavelength conversion-filter element 13 thatconverts laser beams of fundamental waves output from the solid-statelaser element 12 to second harmonics of half the wavelength and has afunction of a birefringent filter.

The semiconductor laser 11 outputs pump light for pumping thesolid-state laser element 12. An emission-side facet 111 of laser beamsof the semiconductor laser 11 is provided to face a facet 123 a of alaser medium 121 of the solid-state laser element 12. It is assumed thatthe semiconductor laser 11 is made of a compound semiconductor materialthat outputs laser beams having a wavelength of 808 nanometers.

The solid-state laser element 12 has a tabular shape, and facets 123 aand 123 b vertical to the optical axis R are in a rectangular shape. Thesolid-state laser element 12 has an optical waveguide structure. Thesolid-state laser element 12 absorbs pump light from the semiconductorlaser 11, forms an inverted distribution state to propagate laser beamsgenerated by induced emission in a direction of the optical axis R, andoutputs linear polarization vibrating from the facet 123 b in apredetermined direction. Specifically, the solid-state laser element 12includes the tabular laser medium 121 that absorbs pump light togenerate induced emission and a clad 122 joined to at least one face ofupper and lower faces of the laser medium 121. The laser medium 121 ismade of a birefringent material (preferably, a material including anoptically uniaxial crystal), and a c-axis (crystalline axis) thereof isarranged in a thickness direction in FIG. 1, one of a-axes (crystalaxes) is arranged in the same direction as the optical axis R, and theother a-axis is arranged in a plane vertical to the optical axis R. Thec-axis matches an optic axis, which is in a direction having only onespeed of light wave (a refractive index) in the material.

Because the oscillating direction of laser beams is the direction of theoptical axis R and there are the a-axis and c-axis of the laser medium121 in the plane vertical to the optical axis R, laser beams moving inthe direction of the optical axis R in the laser medium 121 move in amanner branched into TM (transverse magnetic) polarization (alsoreferred to as extraordinary light) in which a plane of vibration ispresent in a plane formed by the c-axis and the optical axis R and TE(transverse electric) polarization (also referred to as ordinary light)in which the plane of vibration is present in a plane vertical to theplane formed by the c-axis and the optical axis R and includes theoptical axis R. Because a refractive index ne of the laser medium 121with respect to TM polarization and a reflective index no of the lasermedium 121 with respect to TE polarization are different from each otherin a case of the birefringent material, laser beams output from thesolid-state laser element 12 can be linear polarization by using amaterial having a refractive index nc present in a range between ne andno as the clad 122.

It is assumed that the solid-state laser element 12 is formed of thelaser medium 121 including Nd:YVO₄ (refractive index: from ne to 2.17and from no to 1.96 in a wavelength of 914 nanometers), which absorbsthe pump light of 808 nanometers from the semiconductor laser 11 andoutputs laser beams of 914 nanometers, and the clad 122 including Ta₂O₅(refractive index: from ne to 2.08 in the wavelength of 914 nanometers)joined to at least one of the upper or lower surfaces of the lasermedium 121.

Because TE polarization subjected to the refractive index no, which issmaller than the refractive index nc of the clad 122 generated by thelaser medium 121, does not satisfy a total reflection condition on aninterface between the laser medium 121 and the clad 122 due to the abovestructure, TE polarization becomes a radiation mode in which light leaksto the clad 122, thereby causing a large loss. However, TM polarizationsubjected to the refractive index ne, which is larger than therefractive index of the clad 122, in the laser medium 121 satisfies thetotal reflection condition on the interface between the laser medium 121and the clad 122, and is confined in the laser medium 121 to propagatein the optical waveguide in the direction of the optical axis R. As aresult, beams output from the solid-state laser element 12 become linearpolarization (fundamental waves) in a TM mode. That is, the solid-statelaser element 12 outputs the fundamental waves that vibrate in athickness direction (a c-axis direction).

The wavelength conversion-filter element 13 has a tabular shape, andfacets 133 a and 133 b thereof perpendicular to the optical axis R are,for example, in a rectangular shape. The wavelength conversion-filterelement 13 also has a function of a birefringent filter that converts awavelength of a part of the fundamental waves output from thesolid-state laser element 12 to a second harmonic having half thewavelength and filters the remaining fundamental waves traveling backand forth in the wavelength conversion-filter element 13 and enteringinto the solid-state laser element 12. A nonlinear optical materialhaving a cyclic polarization inversion structure such as MgO:PPLN(periodically poled lithium niobate) or PPLT (periodically poled lithiumtantalate) can be used as the wavelength conversion-filter element 13.The wavelength conversion-filter element 13 also has the opticalwaveguide structure, in which a clad 132 having a refractive indexsmaller than that of a nonlinear optical material 131 can be joined toboth or at least one of upper or lower surfaces of the nonlinear opticalmaterial 131 or air can be used as the clad.

It is assumed that hexagonal MgO:PPLN is used as the nonlinear opticalmaterial 131, and Ta₂O₅ having a lower refractive index than that of thePPLN with respect to TM polarization and TE polarization is used on onesurface of the clad 132 and SiO₂ is used on the other surface thereof.Accordingly, because TM polarization and TE polarization having enteredinto the nonlinear optical material 131 of the wavelengthconversion-filter element 13 satisfy the total reflection condition, TMpolarization and TE polarization propagate in the wavelengthconversion-filter element 13 in a waveguide mode.

The c-axis (which is the crystalline axis and also the optic axis) ofthe nonlinear optical material (MgO:PPLN) 131 constituting thewavelength conversion-filter element 13 is expressed below as a z-axisfor distinguishing it from the c-axis of the laser medium 121. Thea-axis (crystalline axis) parallel to the optical axis R is expressed asan x-axis, and a direction vertical to the z-axis and x-axes isexpressed as a y-axis.

The first embodiment is characterized such that the z-axis (crystallineaxis and optic axis) of the nonlinear optical material 131 is arrangedin a plane vertical to the optical axis R with an inclination of angle θwith respect to the c-axis of the laser medium 121 so that thewavelength conversion-filter element 13 has a function as thebirefringent filter as well as the wavelength conversion function. Anexternal shape of the wavelength conversion-filter element 13 is cut ina tabular shape (a hexahedral shape) in a state with the z-axis beinginclined with respect to the c-axis of the laser medium 121. That is,sides of the wavelength conversion-filter element 13 vertical to theoptical axis R are parallel to the a-axis and the c-axis, which are notparallel to the optical axis R of the laser medium 121, and thedirections of these sides do not match the direction of the y-axis andthe z-axis of the nonlinear optical material 131.

An optical film that transmits pump light and totally reflectsfundamental laser beams is formed on the facet 123 a of the solid-statelaser element 12 on the semiconductor laser 11 side, and a reflectionpreventing film that transmits the fundamental laser beams is formed onthe facet 123 b of the solid-state laser element 12 on the wavelengthconversion-filter element 13 side. An optical film that transmits thefundamental laser beams and reflects the second harmonic laser beams isformed on the facet 133 a of the wavelength conversion-filter element 13on the solid-state laser element 12 side, and an optical film thattotally reflects the fundamental laser beams and transmits the secondharmonic laser beams is formed on the facet 133 b of the wavelengthconversion-filter element 13 on a second-harmonic emitting side. Thesetotal reflection films and optical films are formed by laminating, forexample, dielectric thin films.

As described above, the nonlinear optical material 131 has the functionas the birefringent filter as well as the function as the wavelengthconversion-filter element 13, by inclining the z-axis of the nonlinearoptical material 131 of the wavelength conversion-filter element 13 bythe angle θ with respect to the c-axis of the laser medium 121 in theplane vertical to the optical axis R.

An operation of the wavelength conversion laser device is explainednext, focusing on the function as the birefringent filter of thenonlinear optical material 131. Pump light having a wavelength of 808nanometers is output from the facet 111 of the semiconductor laser 11and enters into the facet 123 a of the laser medium 121 in thesolid-state laser element 12. The inverted distribution state is formedin the laser medium 121 by the pump light, and the laser medium 121becomes a mode in which spontaneously emitted light emitted in thedirection of the optical axis R resonates, and the light is amplified byinduced emission. The light travels back and forth between the facet 123a of the laser medium 121 and the facet 133 b of the wavelengthconversion-filter element 13 (optical resonator). However, when a gainby amplification at the time of going around by the optical resonator isbalanced with a loss caused at the time of going around by the opticalresonator, laser beams having a wavelength of 914 nanometers arelaser-oscillated.

Because TE polarization of laser-oscillated laser beams does not satisfythe total reflection condition in the solid-state laser element 12, TEpolarization is lost as a radiation mode, and only TM polarization isoutput from the facet of the laser medium 121. That is, laser beamsoutput from the laser medium 121 becomes TM polarization in which laserbeams are linearly polarized in the c-axis direction.

Laser beams output from the laser medium 121 enter into MgO:PPLN 131 inlinear polarization in the c-axis direction. At this time, because thez-axis of MgO:PPLN 131 is inclined by the angle θ in the plane verticalto the optical axis R with respect to the c-axis of Nd:YVO₄ as the lasermedium 121, linear polarization is separated into TM polarization(extraordinary light) vibrating in a z-axis direction and TEpolarization (ordinary light) vibrating in a y-axis direction, andpropagates in MgO:PPLN 131 while being subjected to different refractiveindexes. Because MgO:PPLN 131 is the wavelength conversion-filterelement 13, MgO:PPLN 131 converts a part of the fundamental waves to asecond harmonic having a wavelength of 457 nanometers, which is half thewavelength of the fundamental waves, and outputs the second harmonicfrom the facet 133 b. The fundamental waves, which have not beenconverted to the second harmonic, are totally reflected by the facet 133b to return in the same route.

In laser beams of the fundamental waves that have traveled back andforth in MgO:PPLN 131, passed through the facet 133 a of MgO:PPLN 131,and returned to Nd:YVO₄ 121, only c-axis components are selected toenter into Nd:YVO₄ 121, and a-axis components become a loss. Forexample, in MgO:PPLN 131, in a case that there is no loss of laserbeams, and a phase difference of laser beams vibrating and propagatingin the z-axis direction and the y-axis direction is zero, beams emittedfrom the facet 133 a of MgO:PPLN 131 are combined and return to theoriginal linear polarization. On the other hand, when a phase differenceoccurs while laser beams travel back and forth in MgO:PPLN 131 or whendifferent losses occur in the respective axial directions at the time ofpropagation, beams emitted from the facet 133 a of MgO:PPLN 131 becomeelliptic polarization or circular polarization. In this case,polarization of the fundamental waves (polarization in the c-axisdirection) is selected by Nd:YVO₄ 121, and components entering in ana-axis direction become a loss.

Specifically, because MgO:PPLN 131 becomes a high-order phase plate, aphase difference of laser beams traveling back and forth in MgO:PPLN 131and vibrating and propagating, respectively, in the z-axis direction andthe y-axis direction is different according to the wavelength. At thistime, a wavelength interval Δλ with smallest loss is expressed by thefollowing equation (1), where λ represents a wavelength of thefundamental waves output by the laser medium 121, Δn represents adifference in refractive index with respect to laser beams vibrating andpropagating in the z-axis and y-axis directions, and L represents acrystal length of MgO:PPLN 131 in the direction of optical axis R:Δλ=λ²/2ΔnL  (1)

For example, in MgO:PPLN 131 having a crystal length of L=4.0millimeters, if the wavelength of laser beams of the fundamental wavesis assumed to be λ=914 nanometers, Δn of MgO:PPLN 131 becomesΔn=ne−no=−0.083452. Therefore, Δλ=1.25 nanometers can be obtained fromthe equation (1). That is, the wavelength having the smallest lossappears periodically for every 1.25 nanometers.

FIG. 2 depicts a relation between a field strength and an axialdirection when laser beams having linear polarization in the c-axisdirection travel back and forth in the nonlinear optical material. Therelation between the field strength and the axial direction (of a singlepass) when electric field E₀ of laser beams output from Nd:YVO₄ 121 andlinearly polarized in the c-axis direction enters into MgO:PPLN 131,travels back and forth therein once, and output from the facet is shownhere. The a-axis and c-axis in FIG. 2 indicate orientation of thecrystalline axis of the laser medium (Nd:YVO₄) 121, and the z-axis andy-axis respectively indicate orientation of the crystalline axis(c-axis) of the nonlinear optical material 131 and orientation of theaxis vertical to the c-axis in the plane vertical to the optical axis R.

Because the strength of the electric field polarized in the c-axisdirection, which enters into MgO:PPLN 131, is E₀, electric fieldcomponents E_(y) and E_(Z) in the y-axis direction and the z-axisdirection immediately after entering into MgO:PPLN 131 are respectivelyexpressed by the following equations (2) and (3), where θ represents anangle formed by the c-axis of Nd:YVO4 and the z-axis of MgO:PPLN 131(hereinafter, “installation angle”):E_(y)=E₀ cos θ  (2)E_(Z)=E₀ sin θ  (3)

When a intensity transmission factor in a single pass of the laser beamsin the y-axis and z-axis directions is, respectively, η_(y) and η_(z),the electric field components E_(y) and E_(z) in the y-axis directionand the z-axis direction, which have entered into MgO:PPLN 131, traveledback and forth therein once, and emitted, are expressed, respectively,by the following equations (4) and (5), derived from the equations (2)and (3).E_(y)′=η_(y)E₀ cos θ  (4)E_(z)′=η_(z)E₀ sin θ  (5)

A calculation using Jones' matrix described later is required for adetailed polarization characteristic, taking wavelength dependency of aloss and the refractive index into consideration. However, the fieldstrength in a condition in which the loss becomes the smallest (that is,a phase difference is zero) or the largest (that is, the phasedifference is n) can be uniquely expressed, regardless of crystalproperties. The electric field components (E_(c)′, E_(a)′) in a casethat a loss of laser beams propagating from Nd:YVO₄ 121 to MgO:PPLN 131and entering again into Nd:YVO₄ 121 is the smallest (a phase differenceis zero) and the electric field components (E_(c)′, E_(a)′) in a casethat the loss is the largest (a phase difference is n) are expressedrespectively by the following equations (6) and (7), derived from theequations (4) and (5).

$\begin{matrix}{\begin{pmatrix}E_{c}^{\prime} \\E_{a}^{\prime}\end{pmatrix} = \begin{pmatrix}{E_{0}\left( {{{\sqrt{\eta_{z}}}^{2}\cos^{2}\theta} + {{\sqrt{\eta_{y}}}^{2}\sin^{2}\theta}} \right)} \\{\left( {{\sqrt{\eta_{z}}}^{2} - {\sqrt{\eta_{y}}}^{2}} \right)E_{0}\cos\;{\theta sin\theta}}\end{pmatrix}} & (6) \\{\begin{pmatrix}E_{c}^{\prime} \\E_{a}^{\prime}\end{pmatrix} = \begin{pmatrix}{E_{0}\left( {{{\sqrt{\eta_{z}}}^{2}\cos^{2}\theta} - {{\sqrt{\eta_{y}}}^{2}\sin^{2}\theta}} \right)} \\{\left( {{\sqrt{\eta_{z}}}^{2} + {\sqrt{\eta_{y}}}^{2}} \right)E_{0}\cos\;{\theta sin\theta}}\end{pmatrix}} & (7)\end{matrix}$

In a case of the phase difference being 0 in the equation (6) and in acase of the phase difference being π in the equation (7), it isunderstood that the loss, that is, components E_(z)′ entering into thea-axis of Nd:YVO₄ 121 increases as a difference between intensitytransmission factors η_(y) and η_(z) in the y-axis and z-axis directionsbecome larger and the installation angle θ becomes closer to 45 degrees.FIGS. 3A and 3B depict installation angle θ dependency of MgO:PPLN of adegree of polarization of laser beams in the c-axis direction afterpassing through MgO:PPLN. A calculation is performed here, designatingthe intensity transmission factor in a single pass of laser beams in thez-axis and y-axis directions, respectively, as η_(z)=0.9 (that is,assuming that a wavelength conversion rate of the single pass in thez-axis direction is 10%), and η_(y)=1.0. The degree of polarization inthe c-axis direction is defined by the strength of polarization in thec-axis direction with respect to a sum of strengths of polarization inthe c-axis direction and the a-axis direction. The strength ofpolarization in each axial direction is proportional to a square of thefield strength in the axial direction.

As shown in FIGS. 3A and 3B, in the case of the phase difference being 0or π, the degree of polarization in the c-axis direction becomes thesmallest when the installation angle θ of MgO:PPLN 131 is close to 45degrees. As shown in FIG. 3A, when the installation angle θ of MgO:PPLN131 is 10 degrees, the degree of polarization in the c-axis direction is0.99964, and when the installation angle θ of MgO:PPLN 121 is 45degrees, the degree of polarization in the c-axis direction is 0.99724.Further, shown in FIG. 3B, in the case of the phase difference being π,the degree of polarization in the c-axis direction becomes substantiallyzero when the installation angle θ is 45 degrees.

Next, the wavelength dependency of the degree of polarization of laserbeams in a case that a loss occurs when laser beams pass through aplurality of materials is obtained by using the Jones' matrix. Jones'matrix J at the time of inclining the z-axis of MgO:PPLN 131 by theangle θ is expressed by the following equation (8), where a rotatingmatrix of angle θ is designated as R(θ), and a phase difference betweenlaser beams in the y-axis direction and laser beams in the z-axisdirection of MgO:PPLN 131 is designated as α(=2π·ΔnL/λ).

$\begin{matrix}{J = {{R(\theta)}\begin{pmatrix}\sqrt{\eta_{z}} & 0 \\0 & \sqrt{\eta_{y}}\end{pmatrix}\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}}\;{\alpha/2}} & 0 \\0 & {\mathbb{e}}^{{- {\mathbb{i}}}\;{\alpha/2}}\end{pmatrix}\begin{pmatrix}\sqrt{\eta_{z}} & 0 \\0 & \sqrt{\eta_{y}}\end{pmatrix}\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}}\;{\alpha/2}} & 0 \\0 & {\mathbb{e}}^{{- {\mathbb{i}}}\;{\alpha/2}}\end{pmatrix}{r\left( {- \theta} \right)}}} & (8)\end{matrix}$

By using the Jones' matrix J in the equation (8), the electric fieldcomponents (E_(c)′, E_(a)′) at the time of entering into Nd:YVO₄ 121after having traveled back and force in the nonlinear optical material(MgO:PPLN) 131 is obtained as shown in the following equation (9).

$\begin{matrix}{\begin{pmatrix}E_{c}^{\prime} \\E_{a}^{\prime}\end{pmatrix} = {J\begin{pmatrix}E_{z} \\E_{y}\end{pmatrix}}} & (9)\end{matrix}$

FIGS. 4A to 4D depict the wavelength dependency of the strength ofpolarization in the a-axis and c-axis directions and the degree ofpolarization in the c-axis direction of laser beams after passingthrough MgO:PPLN. A calculation is performed by using the equation (9),designating the intensity transmission factor in a single pass of laserbeams in the z-axis and y-axis directions, respectively, as η_(z)=0.9(that is, assuming that the wavelength conversion rate of the singlepass in the z-axis direction is 10%), and η_(y)=1.0, and assuming thatE_(z)=1 and E_(x)=0. FIG. 4A depicts a case that the installation angleof MgO:PPLN 131 is 6 degrees, FIG. 4B depicts a case that theinstallation angle of MgO:PPLN 131 is 16 degrees, FIG. 4C depicts a casethat the installation angle of MgO:PPLN 131 is 26 degrees, and FIG. 4Ddepicts a case that the installation angle of MgO:PPLN 131 is 45degrees.

As shown in the degree of polarization in the c-axis direction in FIGS.4A to 4D, as the installation angle θ of MgO:PPLN 131 increases, theboundary between wavelengths in which a loss obtained by the equation(1) becomes the smallest is sharply marked. This is because the strengthof polarization in the a-axis direction increases in a range between thewavelengths in which the loss obtained by the equation (1) becomes thesmallest, and this part becomes a loss at the time of entering into thesolid-state laser element 12.

The largest degree of polarization in the c-axis direction is,respectively, 0.9999, 0.9991, 0.9982, and 0.9972 when the installationangle is 6 degrees, 16 degrees, 26 degrees, and 45 degrees. Further, awavelength width when the degree of polarization in the c-axis directionof laser beams entering into Nd:YVO₄ 121 is 90% (polarization loss is10%) is, respectively, 0.5 nanometer, 0.3 nanometer, and 0.2 nanometerwhen the installation angle is 16 degrees, 26 degrees, and 45 degrees.

FIGS. 5A and 5B are schematic diagrams of a relation between a shape ofthe gain with respect to the wavelength of the fundamental wave and again band of the laser medium. When a fundamental wave w1 having passedthrough the birefringent filter has a shape of the gain as shown in FIG.5A, when laser oscillation occurs at a wavelength position of thefundamental wave w1 overlapping on a peak of a gain band G of the lasermedium 121 (hereinafter, “peak overlapping position”) λ₁, a secondharmonic is generated by the wavelength conversion-filter element 13,thereby increasing a loss at the peak overlapping position λ₁. At thistime, because the gain of the fundamental wave w1 exceeds the lossgenerated by the birefringent filter also in a region R₁ outside of thepeak overlapping position λ₁, laser oscillation occurs, thereby causinglaser oscillation outside a wavelength conversion band T. Laser beamslaser-oscillated at outside the wavelength conversion band T are notwavelength-converted, thereby decreasing wavelength conversionefficiency of the wavelength conversion-filter element 13.

On the other hand, when a fundamental wave w2 having passed through thebirefringent filter has a shape of the gain as shown in FIG. 5B, a gainof the fundamental wave w2 falls below the loss generated by thebirefringent filter in a region R₂ near the outside of the peakoverlapping position λ₁, and an oscillation wavelength width of thefundamental wave w2 is narrowed. Therefore, laser oscillation occurs atthe peak overlapping position λ₁ to generate the second harmonic due towavelength conversion by the wavelength conversion-filter element 13.Even if the loss increases at the peak overlapping position λ₁, laseroscillation does not occur with a wavelength in a region R₂ in which thegain of the fundamental wave w2 near the outside of the peak overlappingposition λ₁ falls below the gain band G of the laser medium.

That is, it is desired to determine the installation angle θ of MgO:PPLN131 so that the shape of the gain of the fundamental wave w2 generateslaser oscillation in a region where the gain band G of the laser medium121 and the wavelength conversion band T overlap on each other and in anadjacent region thereof, but does not generate laser oscillation with awavelength outside the region. More specifically, it is desired to havea shape of the fundamental wave w2 at the time of the installation angleθ being as small as possible, of the installation angles θ at which thegain of the fundamental wave w2 intersects the gain band G of the lasermedium near a point of intersection between the wavelength conversionband T and the gain band G of the laser medium. The installation angle θhaving such a shape of the gain of the fundamental wave is differentaccording to a material and a length of the wavelength conversion-filterelement 13 to be used. Therefore, the most appropriate installationangle θ needs to be determined for each of the wavelengthconversion-filter element 13 to be used. In this manner, the bandwidthof laser beams (fundamental waves) entering into the solid-state laserelement 12 can be narrowed.

In FIGS. 4A to 4B, a peak position with a minimum loss is 914.5nanometers deviated from 914 nanometers, which is a center of thewavelength conversion bandwidth. This is because a calculation isperformed, designating a length of the nonlinear optical material 131 ofthe wavelength conversion-filter element 13 in the direction of theoptical axis R as 4.0 millimeters. When the oscillation wavelength ofthe fundamental wave is set to 914 nanometers, only the length of thenonlinear optical material 131 in the direction of the optical axis Rneeds to be changed. Further, when the wavelength conversion laserdevice 10 is constituted by designating the length of the nonlinearoptical material 131 in the direction of the optical axis R as 4.0millimeters, the peak position of the fundamental wave having passedthrough MgO:PPLN 131 and the position of the wavelength conversion bandof the nonlinear optical material 131 change according to a temperaturechange, and thus the peak positions thereof can be matched with eachother by a temperature adjustment.

For example, the wavelength conversion laser device 10 shown in FIG. 1is held on a heat sink, although not shown, and includes a temperaturedetector that detects the temperature of a thermistor and a thermocouplefitted to the heat sink, a heating and cooling unit such as a Peltierelement or a heater that heats or cools the wavelength conversion laserdevice 10 to a predetermined temperature, and a temperature controllerthat controls the heating and cooling unit so that the temperature ofthe heat sink (the wavelength conversion laser device) detected by thetemperature detector becomes the predetermined temperature.

FIG. 6 is a schematic diagram of a shift of the wavelength conversionband and the fundamental wave in the nonlinear optical material causedby a temperature change. An example in which the length of the opticalaxis R is 4.0 millimeters, and MgO:PPLN 131 is used as the nonlinearoptical material 131 is explained. When the temperature is increased,the wavelength conversion band T (wavelength conversion bandwidth W2)changes at a rate of +0.07 nm/° C. On the other hand, the peak positionof a fundamental wave w3 that travels back and forth in MgO:PPLN 131(output from the birefringent filter) changes at a rate of −0.32 nm/° C.As calculated in the equation (1), the wavelength interval Δλ withsmallest loss is 1.25 nanometers. Therefore, the peak of the wavelengthconversion band T and a peak wavelength of the gain of the fundamentalwave w3 output by the birefringent filter match each other for every1.25/(0.32−0.07)=5° C. Further, a shift of the wavelength conversionband T in MgO:PPLN 131 due to a temperature change by 5° C. is 0.35nanometer (=0.07 nm/° C.×5° C.), which is smaller than about 2nanometers as an oscillation wavelength bandwidth W1 of the gain band Gof the laser medium 121. Therefore, the peak wavelength having thesmallest loss of the fundamental wave w3 output by the birefringentfilter within the oscillation wavelength bandwidth W1 can be made tomatch the peak of the wavelength conversion band T by a temperatureadjustment. During an output of laser beams, the temperature controllercontrols heating or cooling by the heating and cooling unit based on thetemperature detected by the temperature detector so that thepredetermined temperature is achieved.

According to the first embodiment, the nonlinear optical material 131that functions as the wavelength conversion element is formed of theoptically uniaxial crystal, and the optic axis thereof is inclined bythe predetermined angle with respect to the crystalline axis of thelaser medium 121 within the plane vertical to the optical axis R.Accordingly, the nonlinear optical material 131 also functions as thebirefringent filter, and can limit the oscillation wavelength band ofthe fundamental wave that passes through the nonlinear optical material131 and enters into the solid-state laser element 12. As a result, thewavelength conversion efficiency of the wavelength conversion-filterelement 13 can be increased. Further, there is also an effect such thatthe oscillation wavelength band of laser beams can be limited withoutincreasing the number of parts.

Second Embodiment

FIG. 7 is a perspective view schematically depicting a configuration ofthe wavelength conversion laser device according to a second embodimentof the present invention. A wavelength conversion laser device 10A has aconfiguration in which a wavelength conversion element 13A having anonlinear optical material 131A arranged in such a manner that thez-axis (crystalline axis and optic axis) is not inclined with respect tothe c-axis (polarization direction) of the solid-state laser element 12is used instead of the wavelength conversion-filter element 13, and a ½wavelength plate 14 that functions as the birefringent filter isinserted between the solid-state laser element 12 and the wavelengthconversion element 13A, in the configuration of the first embodiment. Inthis case, as in the first embodiment, to limit the wavelength band ofthe fundamental wave entering into the solid-state laser element 12,only an optic axis p of the ½ wavelength plate 14 needs to be inclinedby a predetermined angle within a plane vertical to the optical axis Rwith respect to the polarization direction (crystalline axis c of thelaser medium 121) of the solid-state laser element 12. However, toachieve the same effect as that of the first embodiment, when MgO:PPLN131 is inclined by the installation angle θ, only the ½ wavelength plate14 needs to be inclined by θ/2. In FIG. 7, constituent elementsidentical to those in the first embodiment are denoted by like referenceletters or numerals, and explanations thereof will be omitted. Further,because the operation of the wavelength conversion laser device 10Aaccording to the second embodiment is identical to that of the firstembodiment, explanations thereof will be omitted.

According to the second embodiment, because the optic axis p of the ½wavelength plate 14 is inclined by the predetermined angle with respectto the crystalline axis c of the laser medium 121 within the planevertical to the optical axis R, the wavelength band of the fundamentalwave entering into the solid-state laser element 12 can be limited. As aresult, the wavelength conversion efficiency of the wavelengthconversion element 13A can be increased. Further, although the number ofparts is increased, the ½ wavelength plate 14 has a size in thedirection of the optical axis as small as several tens of micrometersand can suppress an increase in size of the wavelength conversion laserdevice 10 due to an addition of the parts, as compared with a case thatparts of a millimeter order are added.

Third Embodiment

FIG. 8 is a perspective view schematically depicting a configuration ofthe wavelength conversion laser device according to a third embodimentof the present invention. A wavelength conversion laser device 10B has aconfiguration in which a ¼ wavelength plate 15 is provided on asecond-harmonic output side of the wavelength conversion-filter element13, in the configuration of the first embodiment. The ¼ wavelength plate15 is arranged so that an optical axis R thereof is in the samedirection as the c-axis (polarization direction) of the solid-statelaser element 12. However, the facet 133 b on the second harmonic outputside of the wavelength conversion-filter element 13 is processed totransmit both the fundamental wave and the second harmonic, and anoptical film that totally reflects the fundamental wave and transmitsthe second harmonic is formed on a facet 151 of the ¼ wavelength plate15. Constituent elements identical to those in the first embodiment aredenoted by like reference letters or numerals, and explanations thereofwill be omitted.

FIG. 9 depicts wavelength dependency of an intensity of polarization inthe a-axis direction and the degree of polarization in the c-axisdirection of laser beams after passing through MgO:PPLN and a ¼wavelength plate. As in the case of FIGS. 4A to 4D, a calculation isperformed by designating the intensity transmission factor in a singlepass of laser beams in the z-axis and y-axis directions as η_(z)=0.9(that is, assuming that the wavelength conversion rate of the singlepass in the z-axis direction is 10%), and η_(y)=1.0, respectively, andassuming that the installation angle θ of MgO:PPLN 131 is 16 degrees. Asa comparison object, FIG. 4B depicts the wavelength dependency of theintensity of polarization in the a-axis and c-axis directions and thedegree of polarization in the c-axis direction of laser beams afterpassing through MgO:PPLN in a case that the ¼ wavelength plate 15 is notprovided.

As shown in FIG. 9, when the ¼ wavelength plate 15 is provided, a peakinterval of a shape of the gain with respect to the wavelength of thefundamental wave output from the birefringent filter (wavelengthconversion-filter element 13) is doubled as compared with a case thatthe ¼ wavelength plate 15 is not provided as shown in FIG. 4B, and thepeaks are smoothed. Therefore, by providing the ¼ wavelength plate 15,an oscillation band of the fundamental wave can be widened in thewavelength conversion band.

According to the third embodiment, the z-axis (optic axis) of thewavelength conversion-filter element 13 is arranged, inclined withrespect to the c-axis of the laser medium 121 in the plane vertical tothe optical axis R and the ¼ wavelength plate 15 is provided on theoutput side of the second harmonic of the wavelength conversion-filterelement 13. Accordingly, the peaks of the fundamental wave output fromthe wavelength conversion-filter element 13 can be smoothed, and thewavelength band of the fundamental wave can be widened in the wavelengthconversion band. As a result, efficient wavelength conversion can beperformed. Further, in FIG. 7 in the second embodiment, the ¼ wavelengthplate can be provided similarly on the output side of the secondharmonic of the wavelength conversion element 13A to achieve the sameeffect.

In the second and third embodiments, by controlling the temperature ofthe wavelength conversion laser device as in the first embodiment, thepeak wavelength having the smallest loss of the fundamental wave outputby the birefringent filter and the peak of the wavelength conversionband can be made to match each other in the oscillation wavelengthbandwidth.

Furthermore, in the above explanations, an example in which the opticaxis (c-axis) of the laser medium 121 of the solid-state laser element12 is provided in the thickness direction of the tabular laser medium121 has been explained. However, the optic axis (c-axis) of the lasermedium 121 can be provided in any direction so long as the a-axis isarranged parallel to the optical axis R, and the optic axis (c-axis) ofthe laser medium 121 is present in the plane vertical to the opticalaxis R. Also in this case, the optic axis (c-axis) of the nonlinearoptical material 131 of the wavelength conversion-filter element 13 orthe wavelength conversion element 13A or the optic axis p of the ½wavelength plate can be arranged, inclined by a predetermined angle withrespect to the optic axis (c-axis) of the laser medium 121.

INDUSTRIAL APPLICABILITY

As described above, the wavelength conversion laser device according tothe present invention is useful when laser beams having a predeterminedwavelength are efficiently converted to second harmonics.

1. A wavelength conversion laser device, comprising: a solid-state laserelement having a waveguide structure including a laser medium to amplifylaser beams by providing a gain generated due to absorption of pumplight to the laser beams and to output a linearly polarized fundamentalwave; and a wavelength conversion element having a waveguide structureincluding a nonlinear, birefringent optical material in which an opticaxis is arranged, the optic axis being inclined by a predetermined anglewith respect to an optic axis of the laser medium in a plane vertical toan optical axis of the laser beams and selected such that a shape of again of the fundamental wave having passed through the birefringentoptical material generates laser oscillation in a region where a gainband of the laser medium and a wavelength conversion band overlap eachother, the wavelength conversion element being configured to convert apart of the fundamental wave output from the solid-state laser elementto a second harmonic and to differentiate a polarization state of thefundamental wave having passed through the wavelength conversion elementand having entered into the solid-state laser element from the linearpolarization output from the solid-state laser element; and an opticalresonator structure including the solid-state laser element and thewavelength conversion element, the optical resonator structure beingconfigured to resonate the fundamental wave and to output a secondharmonic from the wavelength conversion element.
 2. The wavelengthconversion laser device according to claim 1, further comprising a ¼wavelength plate in which an optic axis thereof is arranged in a samedirection as the optic axis of the laser medium, on a side of thewavelength conversion element that outputs the second harmonic.
 3. Awavelength conversion laser device, comprising: a solid-state laserelement having a waveguide structure including a laser medium to amplifylaser beams by providing a gain generated due to absorption of pumplight to the laser beams and to output a linearly polarized fundamentalwave; and a wavelength conversion element having a waveguide structureincluding a nonlinear, birefringent optical material in which an opticaxis is arranged, the wavelength conversion element being configured toconvert a part of the fundamental wave output from the solid-state laserelement to a second harmonic and to differentiate a polarization stateof the fundamental wave having passed through the wavelength conversionelement and having entered into the solid-state laser element from thelinear polarization output from the solid-state laser element, anoptical resonator structure including the solid-state laser element andthe wavelength conversion element, the optical resonator structure beingconfigured to resonate the fundamental wave and to output a secondharmonic from the wavelength conversion element; and a ½ wavelengthplate arranged between the solid-state laser element and the wavelengthconversion element, an optic axis thereof being inclined by a non-zeropredetermined angle with respect to an optic axis of the laser medium ina plane vertical to an optical axis of the laser beams.
 4. Thewavelength conversion laser device according to claim 3, furthercomprising a ¼ wavelength plate in which an optic axis thereof isarranged in a same direction as the optic axis of the laser medium, on aside of the wavelength conversion element that outputs the secondharmonic.
 5. The wavelength conversion laser device according to claim1, wherein the predetermined angle is selected such that the shape ofthe gain of the fundamental wave having passed through the birefringentoptical material generates laser oscillation in the region where thegain band of the laser medium and the wavelength conversion band overlapeach other and in an adjacent region thereof, but does not generatelaser oscillation with a wavelength outside the region.
 6. Thewavelength conversion laser device according to claim 1, wherein thepredetermined angle is selected as a smallest angle among angles atwhich the gain of the fundamental wave intersects the gain band of thelaser medium near a point of intersection between the wavelengthconversion band and the gain band of the laser medium.